Magnetic induction accelerator



Nov. 23, 1943. p w KERsT 2,335,014

MAGNETIC INDUCTION ACCELERATOR Filed Jan. 13, 1942 2 Sheets-Sheet 1 Fig].

Inventow: Donaid W. Kerst,

by nw z His Abbomwey.

Nov. 23, 1943. w KER 2,33s,'014

MAGNETIC INDUCTION ACCELERATOR Filed Jan. 13, 1942 2 Sheets-Sheet 2 Fig.5

Inventor: Donaki W. Kersh,

His Attorne in the appended claims;

together with further objects and advantages Patented Nov. 23, 19 43 MAGNETIC INDUCTION Accamna'roa Donald W. Kerst, Scotia, N. Y., asslgnor to General Electric Company, a corporation of New York Application January 13, 1942, Serial N0. 426,645 Claims. (Cl. 250--99) The present invention relates to apparatus for accelerating charged particles, such as electrons, by means of magnetic induction effects.

One form of apparatus which has been used for this purpose comprises the combination of a closed vessel and a magnetic system for producing a time-varying magnetic field of such space distribution as to confine electrons projected within the vessel to a circular orbit along which they are continuously accelerated by the field. By this means it proves possible to obtain electrons. of very high energy (of the order of several million electron volts) by the use of a low voltage energizing source.

In usefully applying an accelerator of the type just described, it is necessary to provide means for effectively utilizing the energy of the electrons (or other charged particles) after they have been accelerated to high velocity; for example, by causing them to' impinge on a target so that useful radiations such as X-rays may be obtained. This hasbeen done in one way by abruptly modifying the space distribution of the accelerating field with the end of causing the accelerated particles to deviate from the accelerating orbit andto impinge upon an appropriately positioned target. However, while this procedure has given highly satisfactory results, it requires relatively complicated auxiliary equipment for performing the orbit shifting function.

It is a primary object of the present invention to provide simplified means for utilizing the energy of orbitally moving charged particles produced by a magnetic. induction accelerator.

Inthis connection, an important feature of the invention consists in the provision of a movable target which is normally outside the orbit in which acceleration of charged particles is to occur, but-which may be moved into the orbit after the particles have been fully accelerated, thus intercepting the particles when they have attained maximum energy. In a particular embodiment, the desired motion of the target is accomplished by associating it with a driving means which is energized in synchronism with the cyclical variations of the accelerating field so that the movements of the target and the variations of the field may be precisely correlated, In one case, the target is made of rotary character and is driven by an electric motor supplied from, the samesource as that which suppliesthe accelerating field, while in another embodiment the target is mounted upon a member which is caused to vibrate synchronously with the variations of the accelerating field and in such a direction as to movethe target alternately into and outfof the electron orbit.

The aspects of the invention 'which-I desire to protect herein are pointed out with particularity The'invention itself,

with the drawings in which Fig. 1 is'a partially sectionalized elevation of an accelerating apparatus suitably embodying the invention; Fig. 1a illustrates means for exciting the magnetic structure of Fig. 1; Fig. 2 is a cross sectiontaken on line 2--2 of Fig. 1; Fig. 3 showsdiagrammatically an energizing circuit which maybe employed for controlling the operation of theapparatus of Fig. 1; and Figs. 4 and 5' are fragmentary views illustrating modifications of the invention.

Referring particularly to Fig. 1, there is shown a closed glass vessel in which'defines within its interior an annular chamber ll. As will-be-explained in greater detail at alater point, the vessel l0 provides .a space within which electrons may be accelerated to a high voltage, say on the order of several million volts. The vessel ispreferably highly evacuated. although the presence of a smal1 amount of gas is permissibleunder some circumstances. A high resistance coating, such as an extremely thin layer of silver (not shown), may advantageously be applied to the interior surface of the vessel to prevent wallcharging and the like.

The electron-accelerating mechanism comprises a magnetic structure having generally circular pole pieces which are coaxial with the annular vessel In. These pole pieces include a Pair of juxtaposed circular parts l3 and M which consist, for example, of laminated iron and Wh10h are respectively supported on conically tapered parts l5 and I6. The tapered partsln turn are based upon large cylinders l8 and I9 which connect with closed magnetic cores 2| and 22 so as to provide a complete path for magnetic flux. The elements of the magnetic structure are constituted of ferromagnetic material and should be' manner as to produce a time-varying flux in the magnetic circuit.

The energizing means may appropriately be of the character shown in Fig. 1a which illustrates diagrammatically a portionof the structure of Fig. 1. The coils 24 and 25 are connected in series with each other and with a bank of condensers 32 which are of such capacity as to resonate with the inductance of the coils at a frequency corresponding to the desired frequency of operation of the apparatus. (This may be, for example, on the order of 600 cycles per second, although frequencies differing widely from this value are usable.) To supply the losses of the resonant circuit thus formed, the coils 24 and 25 may be coupled to coils 33 and 34 which are directly energized from an A.-C. power source 36. A relatively small amount of power supplied by the source 38 will serve to maintain the resonant system in excited condition.

Within the closed vessel Ill and also within the region of influence of the magnetic field produced by the pole pieces l and it there is pronecessarily produces an accelerating action on' the electrons.: In this latter respect, the apparatus as a whole consists essentially of a transformer with a secondary comprising a circular path along which the various electrons are accelerated.- Y Although, in general, the voltage per turn'in such a transformer is low, the electrons can achieve very high velocities (e. g., several million volts)- because of the tremendous number of turns which they may execute in their closed path during a single quarter cycle of the field variation.

It has been shown that by a proper design of the magnetic structure the field existing at the electron orbit may be caused to produce a centripetal force in balance with the centrifugal tendencies of the accelerated electrons. In general, this result requires that the following relationship be satisfied:

where is the flux included within the electron orbit, r is the radius of the electron orbit, and H1- is the field strength at the orbit. This equai'ion obviously means that the flux 5 must be twice as strong as that which would be produced by a homogeneous field equal to the field Hr extending over the entire 'area enclosed by the orbital electron path.

'I 'he .condition just specified may be realized by making thev reluctance of the magnetic path greater by an appropriate amount at the electron orbit than its average reluctance within the orbit.

In order to'maintaln fixed proportionality between the enclosed flux and the guide field (i. e.. the field Hr) at all times during the accelerating period, one may include in the magnetic path an air gap or its huivalent. It is readily practicable to control the dimensions of such a gap from point to point over the pole area in such a fashion as to efi'ect the balanced relationship of uide field and enclosed fiux which is desired for the'purpose specified above. The configuration illustrated in Fig. 1 indicates one way in which this may be done.

It is expedient to introduce electrons into the electron orbit in intermittent fashion by anpronriate energization of the electrodes 28, 29 and 3 0. In general, best results are obtained by energizing these electrodes for a brief interval of time at instants when the accelerating magnetic field is near its zero value. This procedure a pears to result in maximum acceptance of electrons by the field producing system and in a maximum yield of high velocity electrons at the end of each accelerating period.

A principal problem in the operation of apparatus of the type under consideration consists in the provision of suitable means for utilizing or realizing the energy of the accelerated electrons afterthey have attained the desired high velocity. However, this is accomplished in a relatively simple manner in accordance with the present invention by providing movable target means for intercepting the electrons at an appropriate point in the accelerating cycle. In the arrangement of Fig. 1, such means comprise a rotating element 40 which is constituted of a pair of similar circular metal segments (suitably of tungsten) supported from a common axis. Accordingly, as the target rotates, it becomes alternately pervious and impervious as viewed at any point near its periphery.

Sincethe electrons are introduced into the accelerating orbit and accelerated in intermittent bursts, as previously explained, fully accelerated electrons exist in the chamber only at spaced intervals of time. It is apparent, therefore, that if the imperforate portion of the target 40 is interposed in the accelerating orbit at appropriate instants, it may be made to intercept the accelerated electrons, with consequent generation of X- rays and other useful radiations. This result can be obtained in one way by driving the target 40 in synchronism with the cyclical variations of the accelerating field. This may be done, for example, by connecting the target to an electrically driven rotor 45 through a shaft 46 in the manner illustrated in Fig. 2. The rotor 45, which is assumed to constitute one elementof a synchronous induction motor, is located within a cylindrical arm 48 extending outwardly at a tangent with respect to the annular vessel I 0. The stator 50 of the motor is outside the arm 48, the stator winding being represented at 5i.

A suitable arrangement for driving the synchronous rotor 45 is illustrated in Fig. 3, in which the elements 28 to 36 correspond to the similarly numbered elements of Figs. 1 and 1a.

The alternating current supply source 36, which energizes the accelerating field producing system, also serves as'a supply source for the stator 50 by which the rotor 45 is driven. Accordingly, the rotor is necessarily operated in synchronism with the variations of the accelerating field. In order to assure the entry of the imperforate target part into the electron orbit only at that point in the accelerating cycle at which the electrons have attained maximum velocity, there is provided in circuit with the stator 50 a phase shifting device 54 by means of which the angular position of the rotor 45 at any given instant may be closely regulated.

Synchronization of the electron injecting means with the other'elements of the system is accomplished by. controlling the excitation of the injecting electrodes 28 to 30 also in accordance with the variations of the accelerating field. This may be done, for example, by means of a peaking transformer 60 which is excited by the variations of the accelerating flux through a coupling 'coil 6 I. The voltage pulses derived from the transformer 60 are impressed upon the grid 64 of a discontinuously conductive discharge device of the transformer 00, the condenser discharges through the device, and potentials are impressed upon the various electrodes 20 to 80, the electrode 80 being rendered sui'ficiently positive to insure injection into the vessel II (Fig. 1) of a burst of electrons at a velocity of the order of several thousand volts. .The injection of electrons is quickly terminated, however, by loss of charge from the condenser 88, continuance of the discharge by current derived directly from the current charging source 'II being prevented by means of a current-limiting resistor Ii. An inductance ll connected in series with the condenser assists in reversing the potential on the anode of the discharge device 08 in order to restore the device to non-conductive condition and to prepare it for further operation at the beginning of the next orbit. In order to assure the functioning of the system in this manner, it is necessary that the target remain outside of the accelerating orbit only over the. intervals ab, a'b', a"b", etc. This means that as-a practical matter the area 'of the perforate portion of the target needs to be only a smal fraction of the area of its imperforate portion. 1

It is clear that the system described in the fdregoingis characterized by a high degree of simplicity, both with reference to the mechanical structures involved and the electrical control means'required. For this reason, the system has the advantage of low cost and ease of maintenance.

Intermittent interception of the accelerated electrons maybe accomplished in another way by an arrangement such as that represented in Fig. 5. In the'latter figure, there is shown a fragmentary portion of a circular vessel 80 of the same general character as the vessel III of Fig. 1. The dotted line B is assumed to represent .the

accelerating orbit of electrons intermittently injected into the vessel and accelerated by means of a cyclically varying magnetic field in conformity with the procedure previously described. In this case, however, the target, which is indicated at 82, is supported at the end of a vibratory member 88 which may consist of a flexible metal ribbon or reed secured at its opposite end to a rigid standard 84. The standard is mounted on a cup-shaped metal member 85 which is sealed into the wall of the vessel 80 as indicated In proximity to the vibratory member 83 there is provided a-U-shaped magnet 86 which is supported from the standard 84 and which has an energizing coil 88 wound upon its central portion.

Current-supply connections for the coil 88 are produced. In circuit between the supp y source ll andthecoil l8 thereisprovi edaphase fting device 81 by means of whic voltage applied to the coil may be adjusted in a desired manner. u

With the arrangement specified the cyclically varying magnetic field produced bythe magnet structure acts on an armature 88 which is secured to the reed 88 in such fashion as to excite the reed to vibratory motion. Assuming the reed to be possessed of an appropriate degree of fiexibility, the amplitude of its vibration niay readily be made such as to carry the target 82 into a position in which it intercepts the electron orbit B. Moreover, by properly phasing the excitation of the coil 88 (i. e., by means of the phase shifting device 01), the instant of electron interception may be made to coincide with the attainment by the electrons of their maximum velocity.

Still another embodiment of the invention is illustrated in Fig. 6 whichshows a fragmentary portion of an accelerating vessel I05 containing electron-intercepting means of a form related to but differing from that of Fig. 5. Within the vessei, a movable target I08 (for example, of tungsten) is supported at the extremity of a vibratory reed I01. The reed in turn is supported through a standard I08 from a metal cup I09 which is sealed into the wall of the vessel.

For the purpose of imparting vibratory motion to the reed I01 reliance is in this case placed upon the action of the magnetic accelerating field rather than upon the provision of separate ex citing means. This. result is obtained by providing in connection with the reed a body of ferromagnetic material III (e. 3.. iron) which is located within the fringes of the accelerating magnetic field. With this arrangement, as the field increases in value, it tends to attract the magnetic element III toward'the interpolar region. The intermittent impulses thus impressed upon the reed I01 set it into vibration and cause the target I08 to be intermittently brought into coincidence with the electron orbit in the manner specified for the target 82 of Fig. 5.

In order to control the vibrations-of the reed I 01 and in particular to adjust the phase of these vibrations with respect to the variations of the accelerating field, there is provided a rod I I3 having its extremity bearing against the lower surface of the reed. The force which is exerted upon the reed by the rod is adjustable by means of a screw-threaded nut H8 cooperating with a correspondingly threaded, outwardly projecting portion of the rod. The rod is connected at an intermediate region to a-flexible metal bellows 5 which is hermetically joined to the cup I01 and which permits the rod to be moved without destroying the vacuum within the vessel I05.

The pressure exerted by the rod on the vibratory reed I01 may be used to tune? the reed to facilitate its vibration at thedesired frequency and also to control the amplitude and phase of its motion. I

What I claim as new and desire to'secure by said orbit at times spaced to cause interception the phase of the of the charged particles after they have been accelerated to high velocity.

2. A magnetic induction accelerator including a vessel within which charged particles may follow a closed orbital path, means for providing charged particles within said vessel, means adjacent to the vessel for cyclically producing a time-varyingmagnetic field of such space dis- ,tribution as to confine charged particles within the vessel to a desired orbit whilecontinuously accelerating them along the said orbit, a target for intercepting the charged particles, and means for cyclically interposing the target in the said o'rbit at times when such target will intercept highly accelerated charged particles.

3. A magnetic induction accelerator including a source of electrons, means for cyclically producing a time-varying magnetic field of such a source oif charged'particles, means for cyclically producing a time-varying magnetic field. of such space distribution as to confine particles from said source to an annular orbit while continuously accelerating them along the orbit, said means including an alternating current supply of fixed frequency, a target which is movable be-- tween a position in which it is wholly outside the said orbit and a position in which it is interposed in the said orbit, and means energized from said alternating current supply for cyclically moving the target between its said two positions synchronously with the cyclical variations of the accelerating field, thereby to cause the target tointercept charged particles at times when they have been accelerated to maximum velocity.

5. A magnetic induction accelerator including a source of charged particles, means for cyclical-- ly producing a, time-varying magnetic field of such space distribution as to confine particles from said source to a desired closed orbit while continuously accelerating them along the orbit, a rotary target having a first rotational position in which it is wholly outside the said orbit and a second rotational position in which it is interposed in the orbit, and means for rotating the target in synchronism with thecyclical variations of the said accelerating field, thereby to cause the target to be interposed in the particle accelerating orbit at times calculated to assure interception of charged particles after they have attained at high velocity.

6. A magnetic induction accelerator including a source of charged particles, means for cyclically producing a time-varying magnetic field of such space distribution as to confine particles from said source to a desired closed orbit while continuously accelerating them along the orbit, a rotary target having a perforate portion and an imperforate portion, and means for supporting the target in proximity to the said accelerating orbit and for rotating the target in synchronism with the variations of the accelerating field,

producing a time-varying magnetic'field'of such space distribution as to conflneparticles from said source to a desired closed orbit while con-' tinuously accelerating them along the orbit, a target which is movable between a position in which it is wholly outside thesaid orbit and a position in which in is interposed in the said orbit, a vibratory member by which the target is supported and by which its motion between the said positions is controlled, and means for inducing vibrations of the said member in synchronism with the cyclical variations of the accelerating field, thereby to cause the target to intercept charged particles when they have attained a high velocity.

8. A magnetic induction accelerator including a source of charged particles, means for cyclically producing a time-varying magnetic field of such space distribution as to confine particles from said source to a desired closed orbit while continuously accelerating them along the orbit, said means including an alternating current supply of fixed frequency, a target which is movable between a position in which it is wholly outside the said orbit and a position in which it is interposed in the orbit, a vibratory member on which the target is supported and by which its motion between the said two positions is controlled, and means energized from said alternating current supply for inducing vibrations of the said member in synchronism with the cyclical variations of the accelerating field, thereby to cause the target to intercept charged particles only when they have attained a high velocity.

9. A magnetic induction accelerator including a source of charged particles, means for cyclically producing a time-varying magnetic field of such space distribution as to confine particles from said source to a desired closed orbit while continuously accelerating them along the orbit, a target, and a vibratory member supporting the target and operable when excited to vibration to move the target from a position wholly outside the said orbit to a postion in which it intercepts the orbit, said vibratory member having a magnetic component which is subjected to the infiuence of the accelerating field in such a sense as to induce vibratory motion of the member in synchronism with the variations of the field, thereby to cause interception of charged particles by the said target only after the particles have attained a high velocity.

10. A magnetic induction accelerator including an evacuated container, a thermionic cathode, means for releasing successive trains of electrons insaid chamber, a time-varying magnetic field whereby said electrons are caused to repeatedly traverse an annular orbit in said container, while being continuously accelerated during traverse, a target which is movable from a position outside of said orbit to a position in said orbit interposed to said electron traverse and means for alternately moving said target from one of said posi- 

